Reaction of 2-methyl-5,6-dihydro-2H-pyran with dichlorocarbene

Dihalocarbene Insertion Reactions into C−H Bonds of Compounds Containing Small Rings: Mechanisms and Regio- and Stereoselectivities,

“…Because of the difference in the magnitude of orbital overlap, the R endo C-H bonds in 8 are more nucleophilic than the corresponding exo C-H bonds. Furthermore, the overlap of the orbitals of the endo C-H bonds at C2 and C5 of 8 with the Walsh orbitals of the three-membered ring is better than the corresponding interaction of the endo C-H bonds at C2 and C4 in bicyclo[3.1.0]hexane (11). This difference in overlap probably accounts for the observed different stereoselectivity of the approaching dihalocarbenes.…”

“…Thus, dichloroand dibromocarbene insert selectively into the C-H bonds adjacent to the three-membered ring of 8. 1 Similarly, dichlorocarbene insertion into the C-H bonds at C2 or C4 of bicyclo[3.1.0]hexane (11) 42 afforded 12a and 12b in 40% yield. The exo product 12b is favored in this reaction (endo:exo ) 1:3).…”

“…In 8, alignment of the σ orbitals of the endo C-H bonds at the R positions to the three-membered ring with the Walsh orbital seems to be better than with the corresponding exo C-H bonds. In bicyclo[3.1.0]hexane (11), the σ orbitals of the exo and endo C-H bonds at C2 and C4 seem to line-up with the Walsh orbitals to roughly the same extent. However, due to steric interactions with the flagpole hydrogen at C3 and the endo hydrogen at C6, preferential attack of dihalocarbenes should take place from the exo side.…”

“…Our recent work shows that dichlorocarbene inserts into norcarane 8, producing as the major product, 9a, which contained the dichloromethyl group endo to the three-membered ring. 1 In contrast, in the reaction of dichlorocarbene with bicyclo[3.1.0]hexane (11), the major insertion product 12b has the dichloromethyl group located exo to the three-membered ring. 1 The magnitude of overlap between the Walsh orbitals and that of the R C-H σ-bond will affect the nucleophilicity of the C-H bond via delocalization of the electrons in the orbitals; the better the orbital alignment, the greater the effect.…”

“…Since then, several different C−H bond insertion reactions of dihalocarbenes have been revealed . A dihalocarbene can insert into a C−H bond that has been activated by the presence of an adjacent heteroatom: oxygen, − nitrogen, or sulfur . In addition, activation using compounds containing π bonds, such as vinyl, − carbonyl, and phenyl 6-8,15b,19 moieties has also been studied.…”